The in utero transplantation of hematopoietic stem cells (HSCs) holds considerable promise as a therapeutic approach to correct a variety of prenatally diagnosed genetic disorders of hematopoiesis and immune function. A number of animal model systems have demonstrated the feasi- bility of transplanting immunologically naive donor cells into preimmune allogeneic and xenogeneic recipients. The transplantation of early gestation fetal liver cells and CD34+ adult bone marrow cells has been associated with long-term donor engraftment and the absence of any serious immunological sequelae. In human-sheep chimeras, sustained human hematopoiesis is observed however, relatively low levels of engraftment of human cells are achieved. These low levels of circulating donor cells may in part be due to the presence of a suboptimal hematopoietic microenvironment for the development of human progenitor cells. This hypothesis is supported by the observation that the administration of human hematopoietic growth factors to human-sheep chimeras significantly increases the number of circulating human cells in the peripheral blood. We postulate that the phylogenetic proximity of nonhuman primates to man will provide a more favorable hematopoietic microenvironment for cell-cell interactions and cross-reactive growth factor activity supporting long- term multilineage hematopoiesis. The optimal tissue source and phenotype of human HSCs for in utero transplantation remains unknown. In addition, the functional activity of the differentiated progeny of engrafted donor cells has yet to be determined. These parameters will need to be defined to facilitate the rational design of clinical trials based on this technology. This proposal will focus on testing the developmental potential of human fetal hematopoietic stem cells following in utero transplantation in a nonhuman primate model system. Specifically we propose to: a) establish long-term human hematopoietic reconstitution of nonhuman primates by in utero transplantation of CD34+ cells from the fetal bone marrow and liver; b) evaluate the developmental potential of phenotypically defined candidate human HSCs in the setting of in utero transplantation and c) determine the immunological function of engrafted human cells in human-primate chimeras. Ultimately, the results of these studies may help to define the optimal source of human fetal HSCs for the development of in utero transplantation protocols for the treatment of genetic diseases in man.